1. Field of the Invention
This invention relates to resin compositions having utility as protective, transparent or translucent coatings for various substrate materials such as wood, paper, metal and plastics.
2. Description of the Art
The field of coating materials has undergone tremendous changes in recent years. One primary concern in the coating industry is the need to lower the emission of volatile hydrocarbons into the air. Under prior practices, coating compositions which are highly viscous were cut with a compatible solvent, such as benzene, toluene or xylene. These solvents aided in processing the coating composition during manufacture and in application to substrates.
In the curing of the coating, that is, the chemical bonding of the various components to form a polymer, the solvent is driven off by the thermal energy used to effect curing. As the thermal curing operation is ordinarily carried out by any number of end consumers, the resultant air quality in the consumer's plant and the atmosphere will depend on the steps which the consumer employs to trap the volatile emissions.
It should also be recognized that a considerable amount of thermal energy is expended in driving off volatile solvents. Aside from cost considerations, the necessary thermal energy to drive off the solvent may cause an undesirable side reaction in the coating or upon the substrate. This problem could be solved by gradually applying the thermal energy to the coated substrate, but with additional time required for the cure. Moreover, the cure rate is essentially independent of the energy required to drive off the solvent. Thus, the cured coating may entrain the solvent, giving rise to a poor coating, e.g. bubbles.
The problem facing the coating industry is, therefore, to provide a resin composition having the desirable properties of former resin compositions, but not containing a volatile solvent which is emitted into the atmosphere during the thermal curing step.
Smith, et al, in U.S. Pat. No. 3,935,330 issued Jan. 27, 1976, suggested a coating composition containing, as required components, a reactive thermoset cross-linker and a reactive solvent which is both thermal and radiation sensitive. Exemplary of the thermoset crosslinkers disclosed in Smith et al are polyfunctional oxiranes, urea/formaldehyde resins and melamine/formaldehyde resins. The reactive solvents are generally described as materials containing both a vinyl polymerizable double bond and a functional group which is reactive under thermal activation.
The oligomers formed through the processing described in Smith et al are fully cross-linked or a penetrated resin composition. That is, the oligomers are a single, continuous complex randomly formed through the common thermosetting functional group. This patent also contemplates the addition of materials which will function to increase the molecular weight of the reactive solvent in the radiation curing step. These latter materials which are called radiation sensitive reactive components will copolymerize through a vinyl radical with the reactive solvent. The resultant copolymer which still maintains its thermal curing functionality then forms a higher molecular weight oligomer with the reactive thermoset crosslinker.
The deficiency in the compositions disclosed by Smith et al lies in the properties of the penetrated oligomer. Specifically, the fact that the oligomer contains at least two thermosetting components and up to two different vinyl containing components leads to a considerable variance in the physical properties of oligomer. That is the variance in the molecular weight of the reactive solvent following the radiation cure must be considered in formulating the end product (oligomer). Similarly, the structure and molecular weight of the reactive solvent must be factored if the additional radiation sensitive component is present to predict the properties of the oligomer. Moreover, the final structure of the penetrated oligomer will vary considerably, if the reactive solvent has not been sufficiently cured by the radiation. That is, the reactive solvent is fully capable of undergoing thermal crosslinking even in the absence of any radiation curing. While this tends to lessen volatility when the radiation cure is incomplete the result is an unpredictably structured oligomer.
U.S. Pat. No. 3,028,367 issued Apr. 3, 1962 to O'Brion teaches the formation of thermosetting resins comprising a copolymer of a hydroxy alkyl acrylate and an alkyl acrylate together with a dihydric or trihydric alcohol and an organic diisocyanate. Organic thiol compounds are shown as useful in controlling the molecular weight of the copolymer portion of the resin. The copolymer which is hydroxy functional is preferably prepared using a free radical liberating agent in the presence dimethylformamide as a solvent. The solvent is removed following the preparation of the copolymer. The final resin composition is then prepared by reacting the copolymer with the alcohol and the isocyanate. Apparently the copolymer is of sufficiently low molecular weight that no solvent is required to lower the viscosity during application to a substrate.
Shur et al in U.S. Pat. No. 3,772,062 issued Nov. 13, 1973 discloses fiber-board coating compositions which are fully cured by actinic radiation. That is, the compositions are penetrating to the extent that all the components contain acrylic groups which are subject to free radical initiated bonding. The use of thermal energy for curing is stated to be avoided by the reference. The difficulty in attempting to use fully radiation cured resins is the lack of physical strength due to incomplete curing leaving a residue of acrylic monomer in the coating. U.S. Pat. No. 3,776,729 to Levy et al issued Dec. 4, 1973 suggest washing off the non-polymerized monomer following the radiation cure. It is easily observed that fully radiation curable compositions are undesirable unless the cure is complete both from a cost and environmental view. Furthermore the inconsistence in the coating which results where not all the monomer has been cured or washed off can produce a product which will discolor or crack when exposed to later sources of radiation, i.e. sunlight.
It is therefore an object of this invention to prepare a curable coating which is essentially free from solvent emission.
It is a further object of this invention to prepare a curable coating wherein all the components are cured.
It is yet a further object of this invention to prepare a curable coating wherein the radiation sensitive reactive diluent and the thermally curable portion of the composition are interpenetrated.
These and other objects of the invention are fully discussed herein. Throughout the specification and claims temperatures are in centigrade degrees and percentages and ratios are by weight unless otherwise indicated.